The problem with which the present invention is concerned is based on the fact that the efficiency of LED chips such as are used for front and rear projection applications, for example, changes firstly in operation depending on the temperature and secondly over the lifetime. If LED chips of different colors are used, the color locus of the summation light formed by superimposition of the light from the individual LED chips changes as a result.
In the case of so-called DMDs (Digital Mirror Device=micromirror device), in the prior art the light emitted by the LED chips is measured in the so-called sump, that is to say at the location to which the light is deflected by the mirrors if it is not intended to be directed toward the outside by the optical assembly. A light sensor that can measure the changes in the light is then fitted there in the sump.
What is disadvantageous about this prior art is that the sensitivity range of the light sensor fitted in the sump has to be co-ordinated with the light emitted by the LED module. If, then, a light sensor that does not appropriately match the LED module is used, this leads to incorrect measurement results. If a control loop that is intended to keep constant for example the brightness and/or the color locus of the light emitted by the LED chips is then intended to be driven on the basis of the signal supplied by the light sensor, reliable functioning of such a control loop cannot be ensured. The same correspondingly holds true if an appropriately matching light sensor is incorporated in the sump in an incorrectly adjusted manner. A laborious readjustment or a complicated adaptation of the control loop used for control is the consequence.